A variety of manufactured goods are available on the market which utilize smooth sheet-like surfaces to enhance the appearance of the product. Representative examples of such goods include sheets for use on trucks and automobiles as body parts, large and small appliances such as washers, dryers, ovens, refrigerators, automatic coffee-makers, and toasters; wall coverings such as wood paneling and laminated sheets; glass or clear plastic plates for use as windows, mirrors, room dividers and the like.
In the past, evaluation of the surfaces of smooth sheets was done by visual observation. In actuality, the human eye is well-designed for use in evaluating surface quality, at least for those surfaces used to enhance product appearance. The reason is that ultimately it is the human eye which will scrutinize the product containing the smooth surfaces in determining whether to purchase that product.
The eye is a good qualitative judge of the acceptability of a particular smooth surface, but reproducible quantitative evaluations of acceptability cannot be rendered. Further, though the eye can distinguish good surfaces from bad, it cannot accurately evaluate surfaces which lie in the range between good and bad.
To address the problem of effectively analyzing smooth surfaces in a quantitative manner, several companies have devised quantitative surface analysis equipment. A representative of Daimler-Benz AG presented a paper to the Society of Automotive Engineers in late February-early March of 1983, entitled "Method for Determining the `Long Term Waviness` of Large SMC Panels". The article discloses the use of a digital length gauge which contacts the surface to be evaluated. Data points corresponding to a vertical deflection of the gauge tip as it passes over the surface to be evaluated are collected and stored. The stored data are then processed to determine an average deflection height, or amplitude, of the line on the surface tracked by the length gauge. The average amplitude along the single line provides a quantitative value which is said to correspond to surface quality. A lower average amplitude corresponds to a better surface.
A second method of quantitative analysis has been developed by ICI Americas, Inc. and is presented in a paper by A. T. Hurst entitled "Measurement Aspects And Improvement Of Surface Profile In Thin Gauge Molded Sheet Molding Compounds", Polym.-Plast. Technol. Eng. 20(1), 65-77 (1983).
The method utilizes a surface wavemeter apparatus which consists of a linear voltage displacement transducer attached to a gauge traversing mechanism which permits the transducer to be drawn over the surface to be evaluated. Both the horizontal and vertical position of the transducer can be determined. The vertical displacement is exaggerated to more clearly show variations on the evaluated surface. The voltage output from the transducer is converted into a digital display and transmitted to a microprocessor.
The transducer evaluates one line on the surface with each complete scan. Generally, two scans are conducted over the object, the lines being at 60.degree. angles to each other in the center of the tested surface. Analysis is performed only in the direction of the visual evaluation.
The digitized voltage output from the scans easily converts to a graphical output which is then analyzed for wavelength, amplitude, wave area, and slope of the ascending line. The analysis requires the presence of at least two troughs which define the endpoints of a wave. Otherwise, calculations and conclusions based on the data are not meaningful. In addition to the need for scanning along the same lines of sight as a human evaluator,it is also important that the scans begin nearest the observer and travel away so that the slope measurement subsequently determined is meaningful in correlating the mathematical values with human rankings.
A third quantitative surface analysis method has been developed by The Budd Company and is described in an article by K. A. Iseler and R. E. Wilkinson, entitled "A Surface Evaluation System For Class A Applications", Society of the Plastics Industry, Inc., 39th Annual Conference, Jan. 16-19, 1984. The Budd Analyzer was essentially the same apparatus as that employed at Daimler-Benz AG, but evaluates the collected data points based on different mathematical concepts. The theory is that the evaluation should not be based on surface waviness, but rather on random surface ripples along a grid line.
Weltlich U.S. Pat. No. 4,476,489 discloses a non-contact surface analysis system which measures the microfinish of a workpiece and compares that finish with known finishes. Weltlich transports the workpiece into a position which permits illumination of the piece, illuminates the surface of the workpiece, views the illuminated surface via a camera, digitizes the signal generated by the camera on a pixel-by-pixel basis, stores the digitized signal, classifies the pixels by varying intensity, and compares the distribution curve of pixel intensity with other curves from preselected finishes. Weltlich generates data points which are utilized only to evaluate the overall reflectance of the analyzed piece and to compare that piece's overall reflectance with those of preselected finishes. The evaluation technique as practiced in Weltlich is also known as the determination of the short-term waviness of the finish. The short-term waviness of a surface correlates to the presence of peaks and troughs of less than about one centimeter on the analyzed surface. Weltlich does not address the analysis of the long-term waviness of a surface.
Other known methods of evaluating surface quality are based primarily on visual inspection, and are qualitative determinations. The visual evaluation of a surface with subsequent ranking has been discussed earlier, and is essentially incapable of being quantified except by statistical analysis of a large number of individual rankings. A variant of the above-mentioned visual-based evaluation method employs a symmetric grid shown onto a dark, polished surface, with subsequent evaluation of the grid lines as they reflect back from the surface. The reflection may be photographed for comparative study. Again, however, this method is qualitative.